4.3 Discussion
PSGWC-bars, with their plain surface and gentle wave type shape, not only help to extend the life of concrete structures, but they also improve the performance of reinforced concrete columns under load when employed as rebars.
The genuine superiority of bond or effective assignation between rebars and their surrounding concrete has an impact on column strength, and when PSGWC-bars are utilised as rebars, this assignation or effective bond between rebars and their surrounding concrete is at its highest.
All the thirty-three columns failed by bursting of concrete. That being so, it may be beneficial to use columns with lateral ties spaced closer than what is specified in codes/ standards. Closer spacing of ties is likely to enhance ductility and energy absorbing capacity of columns. A comparison of the Axial Load (Test) in S. Nos. 7 and 9 in Table 1 and a comparison of the photographs of the corresponding failed columns (Fig. 3) clearly show the benefits of using ties or spirals.
An important objective of the study was to confirm that the usage of PSGWC-bars in columns would not lead to any reduction in the load carrying capacities of columns through any buckling of the rebars. The Axial Load is the average of failure loads of three columns in each type and surface condition, as found in tests.
A comparison of the Axial Load (Test) with the Axial Load (Test) clearly shows the beneficial effect of using PSGWC-bars. In fact, even recognizing that the omission of ties could have lowered the load carrying capacities of columns, it is seen that the columns with PSGWC-bars, but without ties, in S. No. 11 have higher load carrying capacities than the columns with plain round bars but with ties in Sl. No. 1 in Table 1. The higher strength of concrete for the columns in S. No. 11 cannot fully account for the large difference (1510 kN − 1380 kN) in the Axial Load (Test). As can be seen in Note * in Table 1, the usage of PSGWC-bars, coupled with lateral ties, leads to the highest load carrying capacities in columns.
Raising the strength of concrete in S. No. 7 to that in Sl. No. 6 and replacing the grade of steel for PSGWC-bar with the grade of steel in ribbed bars raise the capacity of columns in Sl. No. 7 to 1741 kN which is higher than the capacity of columns with ribbed bars (S. Nos. 4 to 6) with or without galvanizing or with epoxy coating.
The lower Axial Load (Test) of columns with untreated ribbed bars (S. No. 4), compared to the Axial Load (Test) of columns with ribbed bars with galvanizing (S. No. 6), can possibly be explained by the fact that conventional ribbed bars are frequently found to be coated with a layer of rust, as can be commonly seen in India where ribbed bars are quenched as a relatively less expensive means to increase the yield strength of steel.
The axial load carrying capacities of the columns, when corrected for the same concrete strength, show that when PSGWC-bars are used as rebars and nominal ties are used, load carrying capacities of columns tend to be the highest when compared with the ultimate load carrying capacities of columns with other known forms of rebars, whether those rebars may be uncoated, coated or galvanized.
Column 10 in Table 1 shows the additional margins in the load-carrying capacities of columns as the ratios of ultimate capacities (as found in tests) (column 6) to the analytically determined axial loads (column 8) for each type and surface condition of column. As seen in column 10 in Table 1, the use of PSGWC-bars can add to the reserve strength of columns.
The columns, upon failure under axial load (Fig. 3), show that there is no bond between rebars and concrete when such rebars may be given epoxy coatings. As a consequence, test results in column 6 in Table 1 also show conclusively that there is a fall in the load-carrying capacities of columns when the rebars may be provided epoxy coatings.
The results conclusively show that there is no adverse effect on the structural performance of columns when PSGWC-bars are used as rebars. And a comparison of the test results in column 6 for the columns in S. Nos. 8 to 11 shows that instead of lowering the load-carrying capacity, increasing deformations in the wave type configuration lead to greater load-carrying capacities. The tests further show that as in the case of beams, tested at different universities, the use of well-configured PSGWC-bars leads to increased load-carrying capacities of reinforced concrete columns.